1. Field of the Invention
The present invention relates to a vehicle suspension device having a shock absorber of which a damper characteristic is changeable.
2. Description of Related Art
Conventionally, a vehicle suspension device is generally provided with a shock absorber between a sprung portion of the vehicle or the vehicle body and an unsprung portion of the vehicle or the vehicle wheel for damping the vibration of vehicle wheel.
It has been known that the damping characteristic of the shock absorber is changeable, for example, between a hard and a soft, among a multiple steps, or continuously. In the conventional devices, the damping force of the shock absorber is changed so as to obtain a good riding comfort and running stability between the hard and soft damping characteristic.
Japanese Patent Public Disclosure No. 60-248419, laid open to the public in 1985, discloses a control in which the damping characteristic is controlled based on a relative speed and relative displacement of the sprung portion and the unsprung portion. When the directions of the relative displacement and speed of the sprung portion are the same as those of the unsprung portion, it is judged that the damping force of the shock absorber is acting in increasing the vibration. Otherwise, it is judged that damping force is acting in reducing the vibration of the wheel. In this type of suspension device, the damping force is controlled independently on each of the vehicle wheels. As a result, a big difference may occur among the shock absorbers, in the damping forces. This may cause an undesirable change in the steering characteristics of the vehicle or diagonal vibration.